iBook - Memory upgrade

I'm considering upgrading memory on a 700 MHz G3. The system profiler shows 256 MB SDRAM. In slot DIMM1/J12 it shows the 256 card. In the DIMM0/built-in slot it shows empty. My question is, is the computer recognizing the built-in memory? It is performing very slowly and I would think that it would display memory in the built-in slot. A related question is, if the built-in memory is bad, is it possible for built-in memory to be replaced by other than Apple?